Conventional well intervention lubricators, installed on top of subsea Christmas trees, are well known and regularly utilized during operations for “Riser-less Work-overs” or “Light Well Interventions”. The specific purpose of such well intervention lubricators, is to allow a downhole tool deployed from either a wireline, slickline or coiled tubing conveyance member to be lowered into the subsea well, while initially under pressure equalization with the ambient underwater hydrostatic pressure, and then subsequently under pressure equalization with the high pressure well bore fluid.
A well intervention lubricator usually includes a section of a vertical riser pipe with pressure seals and valves at each end. By opening and closing the seals and valves, the pressure inside the lubricator can be adjusted to equalize either with the ambient seawater (to allow the downhole tools to be loaded) or the well bore fluid (to allow the downhole tools to be lowered into the well bore). The lubricator is therefore a kind of pressure “lock-out” chamber. By means of the uppermost pressure retaining seal on the lubricator (which allows the capture and penetration of a conveyance member) having (i.e., being made of) a dynamic nature and permitting the relative movement of the conveyance member, it is then possible to lower the downhole tool into the well to undertake various well operations that may be desired. However, an inevitable side effect of such activities, including the use of such lubricator, is that that the subsea lubricator is likely to become either partially or fully contaminated with well bore fluid.
Examples of such operational experiences are well known, e.g., see the following publications, all incorporated herein by reference:    1. Houot, G., and Issarte, J. P., “Operations Carried Out on a Subsea Wellhead in a Water depth of Approximately 210 Feet”, Society of Petroleum Engineers of AIME, Paper SPE 4827;    2. Clarke, D. G. and Warne, A. S., “Low-Cost Wireline and Logging Operations on a Satellite Well Using a Subsea Wireline Lubricator Deployed From a Dynamically Positioned Monohull Vessel”, Offshore Technology Conference, OTC 5726, May 1988; and    3. Munkerud, P. K., Inderberg, O., “Riserless Light Well Intervention (RLWI)”, Offshore Technology Conference, OTC 18746, May 2007.
The operational examples referenced above, and heretofore known, are limited to the use of lubricators placed in water depths up to approximately 300 m (1,000 ft); that is, operations have to date been restricted to relatively shallow water. In contrast subsea Christmas trees have recently been installed in much deeper water, up to and exceeding 2,000 m (6,600 ft) water depth.
PCT Application No. WO 2009/082234 by FMC KONGSBERG SUBSEA discloses a method and system for circulating fluid in a deepwater subsea intervention stack.
During well intervention subsea lubricator operations there are several pumping activities that are usually required, namely;                pressure integrity testing of the lubricator assembly once it has been installed on top of the well Christmas tree and prior to equalization with well bore pressure conditions;        cleaning out the lubricator following downhole operations by flushing out with a mixture of seawater and chemical additives; and        injection of chemical additives may also be required on some occasions during downhole operations.        
Such pumping operations have until now been performed only from the decks of the support ships or platforms using surface pumps, fluid reservoirs, hydraulic down-lines and return lines. See Houot et al., Clarke et al. and Munkerud et al. for more detail. However the use of such hydraulic down-lines and return lines often creates difficulty, e.g., such lines are large, expensive and quite troublesome to handle even at the relatively shallow water depths described above. One of the main reasons for such difficulties is that the length of these hydraulic lines in conjunction with the desire to retain an economically small diameter thereof results in relatively large frictional losses in the lines during pumping. This leads to inefficiency of the operation. It is expected that these difficulties discussed above for the surface pumps, fluid reservoir, hydraulic down-line and return line apparatus and methods will only be exacerbated in deep water.
Furthermore, the subsea pumping system claimed and described herein is preferably capable of operating under various pressures and flows as required by the specific well profiles and ambient pressure conditions. As such, a variable displacement and variable pressure pumping system should be employed to meet these requirements. Pumps, capable of operating on chemicals used for well operations are currently not available in variable displacement designs. Current art has these pumps connected to variable speed motors. For application in a subsea environment with fixed speed motors, these pumps will not provide the required flow and pressure regulation required for well flushing operations. Alternatively, a hydraulic motor can be attached to these current pumps to provide variable displacement but at the expense of reduced efficiency. Further additional electric motors would be required to provide a power source for these pumps, driving the umbilical to a greater diameter by higher power demand.